Motor vehicle speed

The U.S. Department of Transportation's Federal Highway Administration review research on traffic speed in 1998.[26] The summary says:

  • The evidence shows the risk of having a crash is increased both for vehicles traveling slower than the average speed, and for those traveling above the average speed.
  • The risk of being injured increases exponentially with speeds much faster than the median speed.
  • The severity/lethality of a crash depends on the vehicle speed change at impact.
  • There is limited evidence suggesting lower speed limits result in lower speeds on a system-wide basis.
  • Most crashes related to speed involve speed too fast for the conditions.
  • More research is needed to determine the effectiveness of traffic calming.

The Road and Traffic Authority (RTA) of the Australian state of New South Wales (NSW) asserts speeding (traveling too fast for the prevailing conditions or above the posted speed limit[27]) is a factor in about 40 percent of road deaths.[28] The RTA also say speeding increases the risk of a crash and its severity.[28] On another web page, the RTA qualify their claims by referring to one specific piece of research from 1997, and write "research has shown that the risk of a crash causing death or injury increases rapidly, even with small increases above an appropriately set speed limit."[29]

The contributory factor report in the official British road casualty statistics show for 2006, that "exceeding speed limit" was a contributory factor in 5% of all casualty crashes (14% of all fatal crashes), and "traveling too fast for conditions" was a contributory factor in 11% of all casualty crashes (18% of all fatal crashes).

Driver impairment
Driver impairment describes factors that prevent the driver from driving at their normal level of skill. Common impairments include:

Main article: Driving under the influence
According to the Government of Canada, coroner reports from 2008 suggested almost 40% of fatally injured drivers consumed some quantity of alcohol before the collision
Relative risk of an accident based on
 blood alcohol levels[

Physical impairment
Poor eyesight and/or physical impairment, with many jurisdictions setting simple sight tests and/or requiring appropriate vehicle modifications before being allowed to drive;

Insurance statistics demonstrate a notably higher incidence of accidents and fatalities among drivers aged in their teens or early twenties, with insurance rates reflecting this data. These drivers have the highest incidence of both accidents and fatalities among all driver age groups, a fact that was observed well before the advent of mobile phones.

Females in this age group exhibit somewhat lower accident and fatality rates than males but still register well above the median for drivers of all ages. Also within this group, the highest accident incidence rate occurs within the first year of licensed driving. For this reason many US states have enacted a zero-tolerance policy wherein receiving a moving violation within the first six months to one year of obtaining a license results in automatic license suspension. No US state allows fourteen year-olds to obtain drivers’ licenses any longer.

Old age
Old age, with some jurisdictions requiring driver retesting for reaction speed and eyesight after a certain age.

Sleep deprivation

Drug use
Including some prescription drugs, over the counter drugs (notably antihistamines, opioids and muscarinic antagonists), and illegal drugs.

Research suggests that the driver's attention is affected by distracting sounds such as conversations and operating a mobile phone while driving. Many jurisdictions now restrict or outlaw the use of some types of phone within the car. Recent research conducted by British scientists suggests that music can also have an effect; classical music is considered to be calming, yet too much could relax the driver to a condition of distraction. On the other hand, hard rock may encourage the driver to step on the acceleration pedal, thus creating a potentially dangerous situation on the road.[33]

Combinations of factors
Several conditions can combine to create a much worse situation, for example:

  • Combining low doses of alcohol and cannabis has a more severe effect on driving performance than either cannabis or alcohol in isolation,[34] or
  • Taking recommended doses of several drugs together, which individually do not cause impairment, may combine to bring on drowsiness or other impairment. This could be more pronounced in an elderly person whose renal function is less efficient than a younger person's.[35]

Thus there are situations when a person may be impaired, but still legally allowed to drive, and becomes a potential hazard to themselves and other road users. Pedestrians or cyclists are affected in the same way and can similarly jeopardize themselves or others when on the road.